1. Field of the Invention
The invention relates to an X-ray examination apparatus, comprising a filter which is arranged between an X-ray source and an X-ray detector and which comprises filter members having an adjustable X-ray absorptivity.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from United States Patent Specification U.S. Pat. No. 3,755,672.
The known X-ray examination apparatus comprises a filter for limiting the dynamic range, being the interval between the extreme brightness values, of an X-ray image. An X-ray image is formed on the X-ray detector by positioning an object, for example a patient to be examined, between the X-ray source and the X-ray detector and by irradiating the object by means of X-rays emitted by the X-ray source. If no steps are taken, the dynamic range of the X-ray image may be too large. On the one hand, the X-ray transmittance of some parts of the object is high, for example that of lung tissue; on the other hand, other parts of the object, such as bone tissue, can hardly be penetrated by X-rays. When lead flaps are used to intercept parts of the X-ray beam emitted by the X-ray source in order to shield parts of the object to be examined from the X-rays, the lead flaps are imaged with a uniform, very low brightness. If no further steps are taken, therefore, an X-ray image with a large dynamic range is obtained whereas, for example medically relevant information in the X-ray image is included in brightness variations in a much smaller dynamic range. When the range of brightness values containing medically relevant information is much smaller than the dynamic range of an X-ray image, the X-ray image cannot be readily processed into an image suitable for use as a diagnostic tool. This problem is encountered, for example the X-ray detector is formed by an image intensifier/pick-up chain comprising an image intensifier tube for convening an incident X-ray image into a light image and a video camera for deriving an electronic image signal from the light image. From areas of very high and very low brightness in the X-ray image there are formed areas of very high and very low brightness in the light image. If no further steps are taken, the dynamic range of the light image may exceed the range of brightness values that can be handled by the video camera without causing disturbances in the electronic image signal.
The filter of the known X-ray examination apparatus limits the dynamic range of the X-ray image. To this end the filter comprises a chamber filled with an X-ray absorbing liquid and covered by a movable membrane. The local thickness of the liquid layer in the chamber can be adjusted by means of drive wires which are attached to the membrane and whereby the membrane is locally depressed or lifted. Wherever the membrane is depressed, the local thickness of the liquid layer is reduced and the local X-ray absorptivity of the liquid layer is reduced accordingly; wherever the membrane is lifted, the local thickness of the liquid layer is increased and its local X-ray absorptivity is increased. The chamber areas in which the thickness of the liquid layer can be adjusted constitute the adjustable filter elements. The membrane movement is controlled by servomotors which drive the drive wires. The servomotors are controlled by signals which correspond to local brightness values in the X-ray image or in the X-ray beam. The servomotor control ensures that the drive wires adjust the membrane in such a manner that in parts of the X-ray beam traversing transmissive parts of the object filter elements are adjusted to a high X-ray absorptivity by locally lifting the membrane and that in parts of the X-ray beam which traverse impervious parts of the object, or are intercepted by a lead flap, filter elements are adjusted to a low X-ray absorptivity by locally depressing the membrane.
The filter of the known X-ray apparatus has the drawback that upon local depression and lifting of the membrane the surrounding area of a depressed or lifted part is also depressed or lifted. As a result, local attenuation of the X-ray beam with a high resolution, i.e. with variations of the X-ray absorptivity over very short distances within a cross-section of the X-ray beam, is not possible. A further drawback of the filter of the known X-ray examination apparatus consists in that the membrane is mechanically driven; this precludes fast movements so that a rather long period of time, i.e. several or even some tens of seconds, is required to switch over the setting of the filter. Therefore, the known X-ray apparatus is not suitable for forming a series of X-ray images in rapid succession because in that case the setting of the filter must be changed every time between the successive X-ray images.